VIENNA (AP) ' Nine years ago, the United States invaded Iraq after telling the world that Saddam Hussein had covert weapons programs that could build nuclear arms. In the end, nothing was found. Today, acting on similar fears, Israel is threatening to attack Iran.

While much is known about Iran's nuclear activities from U.N. inspection visits, significant questions remain uncertain, fueling fears of worst-case scenarios and calls for new Mideast military action.

This much shapes the anxieties: In just one decade, Iran's modest nuclear program has expanded into a mature operation that some experts say has the capability to produce a warhead in less than a year.

And this much is verified: Iran has the equipment and raw materials to produce the fissile core of a nuclear weapon, as does any country that can produce its own reactor fuel.

But there is no evidence that the Islamic Republic has taken steps in that direction.

Finally, this much is suspected: The U.N. nuclear watchdog says there are credible indications Iran is researching the intricate technology needed to turn a core into an actual bomb. Tehran denies it, and there's not conclusive proof or any sign it has actually succeeded, but the research alone if confirmed would be seen as clear proof of Iran's intentions.

Reports by the U.N. nuclear agency, the only international organization given firsthand views of Iranian nuclear sites, contain a mix of confirmed data and a variety of theories built on outside intelligence. An Associated Press analysis of the published data by the International Atomic Energy Agency and interviews with officials probes the critical questions on Iran's nuclear ambitions: What can it do and what can't it do now?

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Uranium Enrichment: Core of the Showdown

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Iran's ability to turn uranium into nuclear fuel is at the heart of the confrontation with the West and its allies, which worry that Tehran could push ahead with higher uranium enrichment levels needed for an atomic weapon. Iran says its only aim is to fuel reactors for peaceful energy production and medical research.

Iran is currently running nearly 9,000 centrifuges enriching uranium to produce nuclear fuel, a jump from 8,000 a year ago, according to reports and interviews with officials at the U.N.'s International Atomic Energy Agency. From its main enrichment site, Iran has stockpiled at least 5.5 tons of uranium enriched to 3.5 percent levels, the IAEA says. That is a sufficient level to power reactors, while uranium enriched to at least 90 percent is needed to produce the material for a bomb.

Iran also has churned out 110 kilograms ' nearly 250 pounds ' of 20 percent enriched uranium, which it says is needed to fuel a medical research reactor in Tehran.

In the past three months, Iran has nearly tripled the number of devices producing 20 percent uranium ' nearly 700 centrifuges strung together in four separate series. They can churn it out a rate of about 14 kilograms, or nearly 30 pounds, a month, the IAEA says.

Finally, Iran is building a "heavy water" research reactor, which issues high-pressure fluid as a coolant and can be used with natural uranium as a fuel instead of enriched states of the mineral. The IAEA estimates it could be complete in two to three years and, once in operation, will produce plutonium, another possible pathway to nuclear arms.

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Critical Mass: Enough to Move Toward Weapons?

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Does Iran have enough enriched uranium to move toward the higher-enriched, weapons-grade material?

Judging by the amounts noted in the IAEA reports, the answer is yes. But those reports also say there is no indication that Iran has moved beyond the 20 percent threshold.

The current known amount of 3.5 percent enriched uranium is enough to be turned into cores for four warheads if further enriched.

Enriching enough of it for a single warhead would take about four months with the available centrifuge equipment, says nuclear proliferation expert David Albright.

The confirmed stockpile of 20 percent enriched uranium is about half the amount needed for potential warhead. At the current enrichment pace, it would reach the required amount by the end of the year, according to IAEA figures. The 20-percent material can be converted to weapons-grade much faster than the lower level uranium.

But getting enough weapons-grade material is only part of the equation.

Albright, who regularly briefs U.S. government decision makers and congressional panels on Iran, said any nuclear bomb made in that timeframe would be a crude device with no means of delivery. Mounting it on a missile would take "another six months to a year," he told the AP.

Moreover, with IAEA inspectors on site at the two known enrichment facilities, the world would very likely know when Iran began hiking up the enrichment level to weapons-grade, giving it time to react ' unless Iran has managed to hide a covert site from the view of U.S., Israeli and other spy satellites looking for precisely such a facility.

"Four months is a long time and it's long enough for the international community to respond in a pretty draconian way," said Albright.

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Enrichment Sites: Moving Underground

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Iran's enrichment centrifuges still are nearly all concentrated at the main site in Natanz in central Iran, which is believed to be about 25 feet (six meters) underground and protected by two concrete walls. But it's shifting some operations to a far more heavily fortified site dug into a mountain south of Tehran.

This alone does not violate the U.N. treaty overseeing the spread of nuclear technology, which allows signatory nations like Iran to have enrichment plants. IAEA inspectors have visited the new bunker-like site, known as Fordo, since Iran said it began operations earlier this year.

The Fordo facility, about 40 miles (70 kilometers) south of Tehran, is protected by about 330 feet (90 meters) of rock. It is also surrounded by anti-aircraft batteries and other defenses run by Iran's powerful Revolutionary Guard.

The Fordo site is far smaller than Natanz and only has about 800 centrifuges operating so far, enriching to a 20-percent level. But Iran has recently partially assembled about 2,000 more.

The IAEA last visited Fordo and Natanz last month as part of routine inspections under Iran's commitments to the agency.

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Bomb Design: Parts Required

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Highly enriched uranium or plutonium is only part of the workings of a nuclear warhead. Tests need to be conducted on elements such as containment casings and triggers to start the bomb's atom-busting chain reaction.

The IAEA has no confirmation of such weapons-related work under way in Iran. But the agency has pressed for access to the Parchin military compound southeast of Tehran, where the IAEA suspects high-explosive tests occurred in possible simulations of the blasts needed to set off a nuclear chain reaction.

For nearly four years, it has also asked for ' and been denied ' more information based on 1,000 pages of intelligence- and open-source documentation that suggests Iran drafted computer models of a nuclear warhead as well as worked on developing a nuclear payload for Iran's Shahab 3 intermediate-range missile, which can reach Israel.

U.S. intelligence officials say they generally stand by a 2007 intelligence assessment that asserts Iran stopped comprehensive secret work on developing nuclear arms in 2003. But Britain, France, Germany, Israel and other U.S. allies think such activities have continued past that date, a view shared by the IAEA, which says some isolated and sporadic activities may be ongoing.

On Monday, Iran said it would allow an IAEA inspection of the Parchin military base. But on Thursday IAEA chief Yukiya Amano said his agency has not been officially advised it could do so, and there is no timeframe for a visit.

In Vienna, diplomats said recent satellite images of an Iranian military facility appear to show trucks and earth-moving vehicles at Parchin. They speculated that it was an attempt to scoop up radioactive traces possibly left by tests of a nuclear-weapon trigger but they could not offer any firm evidence. The diplomats spoke on condition of anonymity because they were not authorized to discuss the information on the record.

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Iraq and Iran: Different Pressures

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The Iran showdown has led pundits to make parallels to the prelude to the Iraq invasion. Yet there are significant differences.

With Iraq, the IAEA publicly disputed the existence of such weapons. Yet the Bush White House appeared set on a course toward war.

In contrast, Iran has brought sharp warnings from IAEA about a possible weapons path. President Barack Obama is pushing diplomacy and sanctions on Tehran as an alternative to conflict.

"When I see the casualness with which some of these folks talk about war, I'm reminded of the costs involved in war," Obama said Tuesday in a reference to tough talk by Republican leaders.

"This is not a game," Obama continued, speaking a day after holding talks in Washington with Israeli Prime Minister Benjamin Netanyahu. "And there's nothing casual about it."

The IAEA's most alarmist findings have been based on outside sources, intelligence briefings and documents provided by the United States, European nations, Israel and possibly others.

In its November 2011 report, the IAEA published a 13-page list of suspected experiments it says "indicates that Iran has carried out activities relevant to the development of a nuclear explosive device." It underlined that it found the information "to be, overall, credible," describing it as coming from "a wide variety of sources."

"It is overall consistent in terms of technical contents, individuals and organizations involved and time frames," the report says.

Some critics assert such assessments are overblown and suggest they may be part of U.S and Israeli attempts to justify an attack on Iran. They cite the Bush administration's inaccurate claims that fueled the 2003 invasion and note that warnings that Iran will soon have the bomb go back to the early 1980s, based on U.S.-supported technology provided before the 1979 Islamic Revolution.

Robert Kelley, a former IAEA inspector who was part of a team looking for purported Iraqi nuclear weapons, cites the 2007 U.S. National Intelligence Estimate in arguing against conclusions that Iran now is working on the bomb.

In that assessment, "US agencies concluded 'with high confidence' that Iran had halted its nuclear weapons program in late 2003 under international pressure," he wrote in a January article for the Stockholm International Peace Research Institute. "It is rare for intelligence officials to determine that they have sufficient evidence to say a program has ended, so their information presumably was very good."